That weird because i've only unzip the elgg-2-3-3.zip from elgg and just follow the 6 step from Elgg installation. And that working on my local webserver. Production server => PHP 5.6.31, apache 2.4
We see that append on some pages, not... view reply

No I make a fresh install, just take the zip file install from elgg.org for testing. No 3rd party, only the default bundle plugins. I'm agree with you for the bug in the sender version and the duplicate header, but we have several Elgg... view reply

Sorry for that.
So the problem is that the message doeesn't appear for user. In my log file, there is a duplicated header.
After investigation in Response.php in vendor/symfony there a function :
public function sendHeaders()... view reply

Hello everyone,
We have a problem with system message with the Elgg 2.3.3 versions, don't really know why, but works with Elgg 1.12.16.
On my development environment it's works (Wampserver 3.0.6 on Win10, with Apache...